1. Field of the Invention
The invention relates to a method for producing the flu virus using eggs originating from hens immunized against the flu and also to the use of such a method for the manufacture of a flu vaccine.
2. Summary of the Related Art
Three types of flu virus (A, B and C) are currently known, the type A viruses being responsible for animal and human conditions while the type B and type C viruses are especially pathogenic for humans. The type A viruses are subdivided into subtypes according to the antigenic structure of hemagglutinin (HA) and of neuraminidase (NA) which are the principal glycoproteins of the viral envelope. Sixteen subtypes of HA (H1 to H16) and 9 subtypes of NA (N1 to N9) stand out. The subtype of a type A virus is therefore defined by the HA subtype and the NA subtype which are present in the viral envelope. Wild birds constitute the reservoir of all influenza A subtypes. Certain subtypes of influenza virus type A endemically or epidemically (annual epidemics) infect domestic birds (various subtypes including H5N1 and H9N2), horses (principally H3N8), pigs (principally H1N1, H3N2 and H1N2) and also humans (principally H1N1 and H3N2). Dogs, cats and other wild species can also occasionally be infected with certain subtypes (H3N8 and H5N1 in dogs; H5N1 in cats).
In the veterinary field, poultry farms, and more particularly chicks, chickens, hens and roosters, represent in terms of number the largest population liable to be affected by the flu virus. The avian flu strains of subtypes H5 and H7 may be of two pathotypes: a low path (or LP) pathotype and a high path (or HP) pathotype. The HP strains are responsible for avian flu and derive from the LP strains H5 and H7 after mutations/insertions in particular at the hemagglutinin cleavage site (presence of multiple basic amino acids). Up until now, strict hygiene measures and regular controls are strongly recommended in farms in order to prevent avian flu, and in particular infection with the H5 and H7 subtypes.
In humans, immunization is recommended against the seasonal circulating viral strains responsible for epidemics that are more or less substantial according to the years. Most of the current vaccines are produced using embryonated hen eggs, these eggs being infected with three different flu virus strains (two strains of type A flu virus having the H3N2 and H1N1 subtype and one strain of type B virus). Eggs from hens that have not been immunized against the flu are used in order to prevent any phenomenon of interference which could be harmful to the replication of the virus. It is in fact known that the maternal antibodies are transferred to the chicks after having spent time in the egg and protect them against microbial infections during the first days of life, but, in return, they are responsible for deficient immunity if chicks are prematurely immunized against a microbial agent while protective maternal antibodies still exist against this agent. H. Stone et al. (1992, Avian Dis. 36: 1048-1051) have shown that newborn chicks can be passively immunized against Newcastle disease by inoculating yolk from eggs originating from hens immunized against the Newcastle disease virus (NDV). However, if, following the administration of the egg yolks, the chicks are immunized with the NDV virus, a decrease in the immune response to the vaccine is observed. It is also known, according to the studies by Hamal et al. (2006, Poultry Science 65: 1364-1372), that the rates of transfer of protective maternal antibodies to newborn chicks, in particular the transfer of antibodies directed against the NDV virus or the infectious bronchitis virus (IBV), is between 30% and 40% (percentage of the amount of antibodies in the hen's plasma circulating in the blood of the three-day-old chick), which indicates that a large amount of the maternal antibodies is sequestered in the egg. This is confirmed by the studies of J. R. Beck et al. (2003, Avian Dis. 47:1196-1199), which show that all the eggs contain anti-HA antibodies, approximately 3 weeks after having immunized hens with a strain of inactivated flu virus. Finally, it is known, according to the studies by Fontaine et al. (1963, Pathobiologie, 11/9: 611-613), that if embryonated eggs are inoculated with anti-flu serum, the eggs are protected against infection by the flu virus. All these reasons have led those skilled in the art to consider that if eggs from hens immunized against the flu were used, said eggs would, due to the transfer of the maternal antibodies directed against the flu into the eggs, become incapable of producing flu viruses.
Since the beginning of the 2000s, the economic consequence of avian flu in domestic bird farms has not ceased to increase with the appearance of highly contagious and pathogenic avian virus strains which decimate entire poultry farms. The typing of the HAs of highly pathogenic virus strains shows that almost all of them have the H5 or H7 subtype. It is now feared that virus strains having the H5 or H7 subtype will adapt to humans and may be responsible for a real flu pandemic in humans; serious cases of human flu, admittedly isolated, involving these subtypes have already been reported.
Faced with the risk that the supply of eggs may no longer always be ensured for manufacturing the flu vaccine, new methods of producing vaccines against the flu are currently directed toward the use of cell culture systems.